1. Field of the Invention
This invention relates to circuit breakers with replaceable rating plugs for selectively setting the current at which the circuit breaker trips, and in particular to a microprocessor based circuit breaker with a rating plug which also contains a memory device which records operating data generated by the microprocessor for retrieval by a portable reader when the rating plug is removed from the circuit breaker.
2. Background Information
Circuit breakers are widely used to protect electrical lines and equipment. The circuit breaker monitors current through an electrical conductor and trips to interrupt the current if certain criteria are met. One such criterion is the maximum continuous current permitted in the protected circuit. The maximum continuous current that the circuit. breaker is designed to carry is known as the frame rating. However, the breaker can be used to protect circuits in which the maximum continuous current is less than the circuit breaker frame rating, in which case the circuit breaker is configured to trip if the current exceeds the maximum continuous current established for the particular circuit in which it is used. This is known as the circuit breaker current rating. Obviously, the circuit breaker current rating can be less than but can not exceed the frame rating.
Typically, the circuit breaker current rating is set by a rating resistor which is selected to generate a preset voltage when a current proportional to the maximum continuous current permitted in the protected circuit passes through the rating resistor. In order to provide for adjustment of the current rating so that the circuit breaker can be used to protect circuits with different maximum continuous currents, it is known to incorporate the rating resistor in a replaceable rating plug which may be selectively inserted into the breaker. An example of a circuit breaker with such a replaceable rating plug is disclosed in U.S. Pat. No. 3,826,951.
While circuit breakers have traditionally been electromechanical analog devices, recently circuit breakers have been developed which incorporate a microprocessor to implement the trip logic. An example of such a microprocessor based circuit breaker is disclosed in U.S. Pat. No. 4,351,013. That circuit breaker includes a replaceable rating plug with a resistor selected to provide a reference for the maximum continuous current established for the protected circuit.
Circuit breakers are subject to mechanical wear and electrical damage which determine the service life of the device. It is desirable to know how much useful life remains on a circuit breaker so that the circuit breaker can be replaced before it fails, but yet is not removed prematurely. It is known to attach a mechanical counter to a circuit breaker operating mechanism to record the cycles of operation. While this provides a measure related to the mechanical wear on the circuit breaker, it does not provide an assessment of electrical damage to the circuit breaker. The current interrupted by the circuit breaker can greatly exceed the maximum continuous current under fault conditions. While the circuit breaker will typically be able to operate many times at currents up to, and even slightly above, the maximum continuous current, it probably will only be able to be operated a few times at high fault currents and perhaps even only once in interrupting very high fault currents that could be experienced in a particular installation.
Accordingly, there is a need for a circuit breaker which provides a quantitative measure of service life remaining.
There is also a need for such a circuit breaker in which the quantitative measure of service life remaining is readily available and easily extracted.
There is a particular need for such capabilities in a microprocessor based circuit breaker and further in such a microprocessor based circuit breaker having a replaceable rating plug.
There is an additional need for such a circuit breaker which provides a quantitative measure of the remaining life of the circuit breaker which takes into account the magnitudes of the current interrupted as well as the number of cycles of the operating mechanism.